During the fabrication of a transistor, silicide layers may be used to improve the conductivity of polysilicon. For instance nickel and cobalt silicide (CoSi2, NiSi) may be used as a contact in the source and drain of the transistor to improve conductivity. The process to form metal silicide begins by the deposition of a thin transition metal layer, nickel for instance, on the polysilicon. The metal and a portion of the polysilicon are then alloyed together to form the metal silicide layer.
Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) are the main gas phase chemical process used to control deposition at the atomic scale and create extremely thin coatings. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. ALD process are based on sequential and saturating surface reactions of alternatively applied metal precursor, separated by inert gas purging.
In order to get high-purity, thin, high-performance solid materials on the wafer, the metal-containing precursors are required to have high purity, high thermal stability, and high volatility. Furthermore they should vaporize rapidly and at a reproducible rate, conditions that are usually met by liquid precursors, but not by solid precursors (See R. G. Gordon et Al. FutureFab International, 2005, 18, 126-128).
Available metal precursors (nickel, cobalt, manganese, palladium) suffer either from poor volatility or low thermal stability which results in carbon incorporation in the films during the deposition process. Several Ni precursors such as a glyoximato (M. Becht, et al., J. Phys. IV (1995) C5-465), β-diketonato (T. Maruyama, T. Tago, J. Mater. Sci. 28 (1993) 5345), or cyclopentadienyl nickel (L. Brissonneau, C. Vahlas, Chem. Vap. Deposit. 5 (1999) 135, 143; J.-K. Kang, S.-W. Rhee, J. Mater. Res. 15 (2000) 1828) were used for deposition process. All are solids below 150° C., and their volatilities are very low. High deposition temperatures were needed to obtain nickel films with those precursors. High deposition temperature causes the decomposition of organic ligands resulting in the increase of residual carbon, which often increases electrical resistivity in the deposited Ni film.
Amidinate transition metal precursors are now well described in the literature (See R. G. Gordon et Al. Inorg. Chem. 2003, 42, 7951-7958; R. G. Gordon Chem. Mater. 2010 22, 3060-3066). They have been used for deposition of Ni by CVD. Although volatile, those precursors are usually solids with high melting point (>70° C.) and can suffer from thermal instability (nickel for instance), which is a drawback for the ALD process. On the other hand, bis-cyclopentadienyl precursors are known to be liquid or low melting point solid, and still volatile depending on the substitution on the cyclopentadienyl. For instance, Ni(Me-Cp)2: solid mp=34-36° C., Ni(Et-Cp)2: liquid, Ni(iPr-Cp)2: liquid. However bis-cyclopentadienyl precursors still suffer from thermal stability, with nickel for instance. Alternatively Ni(allyl)(cyclopentadienyl) precursor was used for the deposition of nickel film. However films contain high content of carbon. (T. Kada, Journal of Crystal Growth 275 (2005) e1115-e1119).
Heteroleptic metal precursors containing allyl ligands are relatively scarce in the literature. Nickel and palladium allyl-diketiminate can be prepared according to J. Organomet. Chem. 2009, 694, 667. The compounds are obtained by reaction of the lithium salt of the diketimine with nickel or palladium-allyl-chloride dimer at low temperature. Substituents on the diketiminate are however aromatic, which is not convenient for ALD/CVD application because of resulting low volatility. Nickel(allyl)(pyrroles-2-aldiminate) precursors can also be prepared according to Dalton Trans., 2003, 4431-4436; J. Organomet. Chem. 693 (2008) 3902-3906); WO9830609 (A1). Precursors are also prepared by reacting the lithium salt of pyrroles-2-aldiminate with nickel-allyl-chloride dimer.
As a consequence, a need remains for new transition metal-containing precursors suitable for CVD or ALD process. Desirable properties of the metal-containing precursors for these applications are: i) liquid form or low melting point solid; ii) high volatility; iii) sufficient thermal stability to avoid decomposition during handling and delivery; and iv) appropriate reactivity during CVD/ALD process.